Vehicle manufacturers are today striving to increase engine efficiency and reduce fuel consumption. This is specifically an issue for manufacturers of heavy vehicles, such as trucks and buses. One way of improving engine efficiency and fuel consumption is waste heat recovery. In vehicles with combustion engines some of the energy from the fuel is dissipated as heat through the exhaust pipes and the engine cooling system. By the use of a waste heat recovery system the heat from the exhaust gases may instead be used for example to heat various vehicle components or to produce mechanical work. Such mechanical work may for example be transferred to the powertrain and thus be used to propel the vehicle.
A waste heat recovery system typically comprises at least one heat exchanger transferring heat between a heat source, such as exhaust gases, and a working fluid. The heat transfer between the working fluid and the heat source is an exchange of energy resulting in a change in temperature. A waste heat recovery system may also be based on a Rankine cycle and thus comprise a working fluid, a pump for circulating the working fluid in a circuit, at least one evaporator (heat exchanger), an expansion device and a condenser. The working fluid in such waste heat recovery system is suitably in a liquid state to start with. The pump pressurizes the working fluid which is pumped through the evaporator. The working fluid is heated by for example exhaust gases led through the evaporator and the working fluid thereby evaporates. The vapour may subsequently be expanded in the expansion device. By means of the expansion device the recovered heat may thereby be converted into mechanical work. The vapour is thereafter cooled in the condenser, such that the working fluid is brought back to its initial liquid state. The condenser is thus typically connected to a cooling system, which could be part of the engine cooling system or a separate cooling system.
The operating temperature of waste heat recovery systems is normally quite high. The thermal inertia of the system could result in a high temperature even after the system has been shut down. Too high temperatures could damage the working fluid and other components of the waste heat recovery system. It is therefore important that the waste heat recovery system is cooled down before being shut down.
Document JP2009173124 A describes a method for avoiding overheating of an engine cooling medium used in an exhaust heat recovery device in a hybrid vehicle. The exhaust heat is used to heat the cooling medium by means of a heat exchanger. If the temperature of the cooling medium is above a threshold temperature when the vehicle is stopping and the combustion engine is turned off, the combustion engine is driven by an electric machine to an idling speed, such that the exhaust gas with a lower temperature is cooling the heat exchanger and thus the cooling medium.